A method and a device of this type are known from DE 40 27 990 C1.
At least an embodiment of the invention relates to a so-called laser scanner that is designed to measure a spatial region and/or an object three-dimensionally. Such a laser scanner is described e.g. in DE 103 61 870 A1. This known laser scanner has a measuring head that can be rotated about a vertical axis. The measuring head contains a rotor that can be rotated about a horizontal axis. The rotor emits an emission light beam and receives a reception light beam reflected from an object. (Reflection within the meaning of the present invention need not necessarily be a total reflection, but rather can also be a diffuse reflection or scattering of the emitted light beam.) The distance between the measuring head and the object is determined from the propagation time of the emission light beam and of the reception light beam. The rotation of the rotor and of the measuring head makes it possible to move the emission light beam by 360° in Azimut and by approximately 270° in elevation. In this way it is possible to measure virtually the entire space all around the known laser scanner. Typical applications for such laser scanners are the measurement of buildings (inside and/or outside), tunnels or the measurement of large objects such as ships' hulls, for instance.
The propagation time of the emission and reception light beams can be determined in various ways. In principle, a distinction is drawn between pulse propagation time methods and CW (Continuous Wave) methods. In the pulse propagation time methods, the emission light beam contains only a short emission pulse for each measuring operation. The time until the reflected pulse arrives in the receiver is measured. In the CW methods, an (at least substantially) continuous emission light beam is emitted and the propagation time is determined on the basis of a phase shift between the emission and reception light beams. In this case, the emission light beam is typically amplitude-modulated with the aid of a modulation signal and the phase shift of the modulation signal in the emitted and received light beams is used for determining the propagation time. The higher the modulation frequency, the more accurately the distance can be determined here. However the unambiguity range decreases as the modulation frequency increases; since the phase shift between emission and reception light beams is repeated after a phase cycle of 360°.
DE 40 27 990 C1 cited in the introduction therefore proposes a distance measuring device with a modulated emission light beam according to the CW method, wherein the emission light beam is amplitude-modulated with a rectangular-waveform modulation signal having a first, relatively high modulation frequency, and wherein said emission light beam is interrupted after a specific number of periods of the modulation signal for a relatively long time period. This interruption can be interpreted as amplitude modulation with a second modulation signal having a second, lower modulation frequency. In other words, the emission light beam in this case is amplitude-modulated with a first, higher and with a second, lower modulation frequency, wherein the two different modulation frequencies determine the unambiguity range. The latter is significantly larger than when just one modulation frequency is used.
In DE 43 03 804 A1 the method according to DE 40 27 990 C1 is deemed to be disadvantageous insofar as the emission light intensity averaged over the duration of the entire signal period is reduced by the amplitude modulation with the lower, second modulation frequency. This is said to lead to a reduction of the signal/noise ratio and, consequently, to have the effect that objects having a low reflectivity can no longer be measured. In order to avoid this disadvantage, DE 43 03 804 A1 proposes modulating the emission light beam alternately with the higher first and the lower, second modulation frequency, that is to say that in each time interval the emission light beam is modulated with only one of the two modulation frequencies in each case. However, this method results in lengthened measuring times since each object has to be measured twice. The higher measuring time is disadvantageous particularly in the case of a laser scanner, because the emission light beam can then only be pivoted relatively slowly.